1. Field of the Invention
The present invention relates to the art of flue gas desulfurization and more particularly, to a method for controlling the concentration of a slurry in a wet flue gas desulfurization apparatus in which SO.sub.2 in the exhaust gas is absorbed in a slurry of suspended solid matters and the solid sulfur compounds are recovered as byproducts.
2. Description of the Prior Art
At present, the mainstream of flue gas desulfurization systems is a flue gas desulfurization apparatus using a so-called wet lime process in which the flue gas is desulfurized using CaCO.sub.3 or Ca(OH).sub.2 as a absorbent in order to recover sulfur in the form of calcium sulfite or calcium sulfate (gypsum). For instance, this process is described in detail in Japanese Laid-open Patent Application 57-63117 and other numerous publications.
Reference is now made to FIG. 1 illustrating a currently, industrially, widely adopted flue gas desulfurization apparatus using wet lime processes.
Flue gas 1 comprising SO.sub.2 is passed into a body of an absorption tower 2. At the lower portion of the absorption tower 2 is provided a tank 3 containing a slurry in which a Ca compound is suspended. The slurry is agitated by means of an agitator 4 to prevent the solid matters from settling. The slurry suspending the Ca compound is fed to the top of the tower 2 by means of a circulation pump 5, by which it is sprayed throughout the tower and flows down through the tower while contacting the flue gas, before being returned to the tank 3. The flue gas from which SO.sub.2 has been removed by contact with the slurry is discharged through a mist eliminator 6 as a purified gas 7. On the other hand, to the tank 3 is fed a slurry of CaCO.sub.3 or Ca(OH).sub.2 through line 8 in an amount which depends on the amount of SO.sub.2 being absorbed. The slurry which contains calcium sulfite produced by the absorption of SO.sub.2 with the absorbent is fed from line 9 to an oxidizing column 10. Air 12 is blown from a bubble generator 11 provided at the bottom of the oxidizing column and sulfuric acid is fed from line 13, whereby calcium sulfite as well as unreacted CaCO.sub.3 or Ca(OH).sub.2 is oxidized into gypsum. The gypsum slurry from the oxidizing column 10 is passed through line 14 into a thickener 15 and the resulting concentrated gypsum slurry is fed through line 16, a tank 17 and a pump 18 into a centrifugal separator 19 to obtain gypsum 20. The filtrate is fed to a tank 21 and then through a pump 22 and line 23 into the thickener 15. The supernatant liquid in the thickener 15 is passed from line 24 to a tank 25 and may be used in adjustment of, for example, the absorbent in the flue gas desulfurization apparatus or may be discharged from a pump 26.
In view of these circumstances, the present inventors made intensive studies to simplify the existing flue gas desulfurization apparatus for economy. As a result, it was found that by analysis of experimental data of the velocity of reaction between crystals of CaCO.sub.3 or Ca(OH).sub.2 and SO.sub.2, the oxidation reaction velocity of calcium sulfite produced by absorption of SO.sub.2, the rate of settle of gypsum, the underlying concept of the known `one operation in one step` process in which the respective reactions were carried out in individual steps could be overcome. More particularly, operations including absorption and oxidation of SO.sub.2, settlement and concentration of gypsum and recovery of supernatant liquid could be collectively carried out in an absorption tower having a tank in a simple and collective manner by proper control of concentrations of slurries in the desulfurization system.
Accordingly, an object of the invention is to provide a method for controlling a concentration of a Ca compound in slurry in a wet flue gas desulfurization apparatus.
The above and other objects, features and advantages of the present invention will become apparent from the following detailed description with reference to the accompanying drawings.